Deuteronomy 27:20

Accursed is he who lieth with his father’s wife, because he uncovereth his father’s skirt. And all shall answer together, and say, Amen.

BECAUSE HE HATH UNCOVERED HIS FATHER’S SKIRT. This is a respectful euphemism for the father, for in the Torah He mentioned, he hath uncovered his father’s nakedness. So also He mentioned the term “their nakedness” in the cases of his brother’s wife, Verse 21. and his uncle’s wife, Verse 20. for there the transgressors are liable to severe punishment, death [by the court, if there were witnesses] or extinction [if there were no witnesses] and Scripture thus needed to condemn the evil they had done; therefore it mentioned “nakedness” concerning them. But here the transgressor being “cursed,” it was sufficient to mention because he hath uncovered his father’s skirt, for he is deserving of an imprecation merely for uncovering his father’s skirt, for it disgraces his honor, and anyone who dishonors his father, is cursed. Similarly, A man shall not take his father’s wife, and shall not uncover his father’s skirt is a respectful euphemism meaning that he should not approach her, for it is not proper to uncover the skirt which he spread over a wife by means of a canopy, similar to what is stated, spread therefore thy skirt over thy handmaid. This excludes the interpretation of him who says — above 23:1] as referring to a woman violated by his father. To this Ramban objects, for this opinion coincides with that of Rabbi Akiba, while the majority of the Sages hold that a man may marry such a woman (Yebamoth 49a). that this verse [A man shall not take his father’s wife] includes a woman that was violated by him.